This invention relates to new hydroxyl-terminated N,N-disubstituted oligourethanes and polyurethanes, to a process for their preparation, and to their use in the production of plastics.
N,N-Disubstituted urethanes are known to show greater thermal stability and resistance to hydrolysis than N-mono-substituted urethanes. Since N,N-disubstituted urethanes cannot be obtained by polyaddition of diols to diisocyanates, they are generally produced by polycondensation of bischlorocarbonic acid esters with disecondary amines in the presence of bases. See Houben-Weyl, Vol. E20, Part 2, Thieme-Verlag, Stuttgart, pages 1580, 1710. Disadvantages of this process include the limited availability and the comparatively high price of the disecondary amines, as well as the poor reproducibility in obtaining the average molecular weights required. In addition, this process yields only amino-terminated products that for many applications have undesirably high reactivity with isocyanates. Accordingly, there has until now been little technical interest in N,N-di-substituted urethanes.
The object of the present invention is to develop an economic process for the production of N,N-disubstituted urethanes which avoids the described disadvantages.
It has now been found that hydroxyl-terminated N,N-disubstituted polyurethanes can be produced by reaction of isocyanate-terminated compounds with oxetane methanols optionally substituted in the 3-position; complete or partial alkylation of the resulting urethane- and oxetane-group-containing compounds at the urethane nitrogens by the reaction with alkylating agents in the presence of metal hydroxides; and, finally, formation of terminal hydroxyl groups by exposing the oxetane groups to strong acid to induce ring opening. The resultant urethane-containing compounds, because of the multiplicity of hydroxyl groups, are useful as chain extenders or crosslinking agents in the preparation of polyurethanes and other isocyanate-based plastics.
U.S. Pat. No. 4,413,112 and 4,472,550 disclose certain oxetane-containing urethanes used as emulsifiers in the production of molded articles containing lignocellulose. The disclosed compounds include only linearly extended structures and not the branched structures of the present invention.
British Patent 1,324,527 discloses certain compounds having the general formula ##STR2## wherein R.sup.1 can be, inter alia, various hydrocarbon groups; R.sup.2 can be, inter alia, various hydrocarbon groups or CH.sub.2 --OH; and Hal is a halogen. The group R.sup.1, however, is never described as a multivalent group having more than one substituent, as is required for the present invention. Consequently, compounds of the reference in which R.sup.2 is a hydrocarbon group cannot serve as chain extenders or crosslinkers because such compounds have only one hydroxyl group. Compounds of the reference in which R.sup.2 is CH.sub.2 --OH have two proximately located hydroxyl groups and can thus serve as chain extenders, but, because the R.sup.1 group is never multivalent, such compounds never have distally located hydroxyl groups and must, therefore, be inherently different chain extenders from the compounds of the present invention.
The British Patent discloses a process for preparing the disclosed compounds by ring-opening an oxetane-ring precursor using a method similar to that used in the present invention. Unlike the present invention, however, the reference does not disclose or suggest how N,N-disubstituted urethane embodiments could be prepared. It has now surprisingly been found that the N-alkylation or N-arylation of oxetane-containing N-monosubstituted urethane intermediates occur smoothly without significantly cleaving the urethane ester bond or ring-opening the oexetane ring.